Computer-assisted surgery tools and system

ABSTRACT

A surface digitizing tool has a film of micro-sensor elements, with each micro-sensor element related in a network affected by a shape of the film. The film is flexible to conform to a shape of a selected surface of an object. A processing unit receives signals from the micro-sensor elements of the network. The processing unit has a model generator producing a model of the selected surface of the object from the signals of the network of resistive elements. A positioning frame aligns a position and orientation of a drilling tool with respect to a bone element.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims priority on U.S. Provisional PatentApplication No. 60/821,331, filed on Aug. 3, 2006. This patentapplication incorporates by reference U.S. patent application Ser. No.11/339,499, by the present assignee, published as United StatesPublication No. 2006/0189864.

FIELD OF THE APPLICATION

The present application generally relates to Computer Assisted Surgery(CAS) such as hip joint resurfacing surgery and, more precisely, to amethod for assisting hip joint resurfacing surgery and like orthopaedicsurgery with CAS systems.

BACKGROUND OF THE APPLICATION

Orthopaedic surgery is constantly evolving to lessen the effects ofsurgery on patients. In order to reduce the amount of post-surgicalpain, new methods and tools have been developed in CAS to minimize theinvasiveness of surgery. Moreover, CAS systems constantly involve newfeatures to accelerate surgeries.

Also, CAS is more commonly used in surgical rooms, so as to provideprecision and accuracy to the surgeon. By way of CAS, position andorientation information is gathered during the surgical procedures, soas to provide to the surgeon real-time visual/digital data about bonealterations, tool navigation, and surgical parameters.

One of the issues pertaining to the efficiency of CAS is the creation offrames of references and the digitization of bone models. In such cases,a plurality of points are digitized on the bone elementsintraoperatively, which represents a time-consuming operation.

Hip joint resurfacing surgery involves the introduction of hip jointcomponents in a patient. The acetabulum and the femoral head areresurfaced so as to receive an acetabular cup implant and a femoral headimplant, respectively. The femoral head implant consists of a ball headreceived at an end of the resurfaced femoral head. Therefore, theimplanted femoral head and the cup (i.e., acetabular or pelvic implant)coact to create the artificial hip joint. In comparison with total hipjoint implanting surgery, the hip joint resurfacing surgery removes arelatively small amount of bone while preserving joint stability.

Different output values are of concern in hip replacement surgery. Inorder to reproduce a natural and/or improved gait and range of motion toa patient, the position and orientation of the implants, the offset ofthe femur and the limb length must be considered during surgery. Thework of the surgeon during hip replacement surgery will have a directeffect on these output values.

Known hip joint resurfacing surgery techniques presently involvespecific tools so as to obtain precise position and orientation for theimplants. As various types of reamers are used to resurface the femoralhead, a plurality of alignment steps are performed to align the toolswith the cuts to be made. It is, for instance, of nonnegligibleimportance that the femoral neck not be damaged (i.e., notched) by thereamers, to prevent fracture-prone weaknesses in the femoral head.Moreover, the resurfacing must be as precise as possible, for instance,to reduce the amount of cement required for implanting the ball headimplant to the resurfaced ball head.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a novel tool and systemfor digitizing bone surfaces in computer-assisted surgery.

It is a further aim of the present invention to provide a novelpositioning frame for adjusting a position and orientation ofbone-altering tools in computer-assisted surgery.

Therefore, in accordance with the present invention, there is provided amodel generator system for generating models of objects, comprising: asurface digitizing tool having a film of micro-sensor elements, witheach micro-sensor element related in a network affected by a shape ofthe film, the film being flexible to conform to a shape of a selectedsurface of an object; a processing unit for receiving signals from themicro-sensor elements of the network, the processing unit having a modelgenerator producing a model of the selected surface of the object fromthe signals of the network of resistive elements.

Further in accordance with the present invention, there is provided apositioning frame for aligning a position and orientation of a drillingtool with respect to a bone element, comprising: a connector portionadapted to releasably grasp the bone element; a support portionoperatively connected to the connector portion and positioned withrespect to a portion of the bone element to be drilled; an alignmenttube operatively connected to the support portion, the alignment tubebeing adapted to receive a working end of a drill to align the drillwith the bone element; and joints between the alignment tube and theconnector portion to adjust a position and an orientation of thealignment tube with respect to the bone element.

Still further in accordance with the present invention, there isprovided a method for drilling a bone element in computer-assistedsurgery, comprising: providing a positioning frame having a drill guidewith adjustable degrees of freedom for the drill guide in thepositioning frame, and a computer-assisted surgery system providingorientation data associated with tracking of the drill guide and of aframe of reference of the bone element; clamping the positioning frameto the bone element such that the drill guide is in the vicinity of aportion of the bone element to be drilled; displacing the drill guidealong the degrees of freedom as a function of the orientation data fromthe computer-assisted surgery, until a desired orientation is reachedfor the drill guide; and drilling the bone element by passing a drillbit through the drill guide in the desired orientation.

Still further in accordance with the present invention, there isprovided a method for generating a model of an object comprising:providing a film of micro-sensor elements, with an individual positionin the film of each said micro-sensor element known; obtaining a signalfrom each said micro-sensor element when the film is shaped to model anobject; determining a shape variation of the film at each saidmicro-sensor element from the signal; and generating a model of theobject from the shape variation and the individual position of each saidmicro-sensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevation view of leg bones involved in a hipreplacement method;

FIG. 2 is a flowchart of a method for hip joint resurfacing surgery;

FIG. 3 is a schematic view of a surface digitizing tool in accordancewith a preferred embodiment of the present invention;

FIG. 4 is a block diagram of a hip resurfacing CAS system having a modelgenerating system in accordance with another preferred embodiment of thepresent invention; and

FIG. 5 is a perspective view of a positioning frame mounted to a femurin accordance with another preferred embodiment of the presentinvention; and

FIG. 6 is a schematic view of a pattern of micro-elements as used in oneembodiment of a flexible film of the surface digitizing tool of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the drawings, and more particularly to FIG. 1, bones of theleg that will be involved in the hip replacement surgery of the presentembodiment are generally shown at 1. FIG. 1 is provided as reference forthe description of the steps of the hip replacement surgery methoddescribed herein. The bones are the pelvis 10, the femur 20, the tibia30 and the fibula 40. Hereinafter, parts of these bones will each bereferenced by numerals from the same numeric decade. For instance, partsof the pelvis (e.g., the acetabulum 11) will bear reference numeralsbetween 11 and 14.

Referring to FIG. 2, a method for hip joint resurfacing surgery inaccordance with the present embodiment is generally shown at 100.Although the method 100 is referred to in the singular, various choicesof procedure will be given to the surgeon, as will be set forth in theforthcoming description, according to the preferences of the surgeon. Aplurality of sequences can be derived from the method 100 according tothe decisions of the surgeon.

In Step 102, preparative steps for surgery are effected. Namely, generalpatient information can be entered into a CAS system for opening apatient file. For instance, a general patient profile can be entered,consisting of the name, birth date, identification number, sex and thelike, the side to be operated, as well as more specific data pertainingto the surgery, such as leg length discrepancy (with the identificationof the longer leg), if applicable, and parameters to define the flow ofthe application and the display. For instance, the leg lengthdiscrepancy is measured using X-rays of the hip joint. More precisely,the leg length discrepancy is measured from the vertical comparisonbetween the lesser trochanters. These X-rays are typically taken duringthe diagnostic stages leading to surgery, so they are usually availablefor hip joint surgery. Alternatively, X-rays may be taken as part ofStep 102. It is also contemplated to import DICOM files or digitalX-rays.

It is pointed out that the general patient information can be enteredpreoperatively. Moreover, the entering of the general patientinformation is straightforward such that the surgeon need not beinvolved. However, in order to minimize the preoperative procedures,actions of Step 102 can be performed at the beginning of the surgicalsession, during the short time span preceding the surgery.

Other values that will potentially be considered in the method 100 areinclination and anteversion for the pelvic implant, CCD (collodiaphysealangle) and anteversion for the femoral implant.

The calibration of the various surgical tools to be used is done. Forinstance, a calibration base and method, as set forth in U.S. Pat. No.6,996,487 by Jutras et al., can be used for the calibration. Also,correspondence between the tracking of the tools and the display on aCAS system can be verified in further calibration steps included in Step102. A permanent calibration system can also be used, as set forth inInternational Publication No. WO 2005/102202.

Surgery is initiated between Step 102 and subsequent Step 104, by thesurgeon exposing the hip joint. No computer assistance is requiredthereat.

In Step 104, the trackable references are secured to the pelvis with apelvic modular reference, and to the femur with a femoral modularreference. The pelvic modular reference can be inserted in a cranial orlateral position. Alternatively, the trackable references may be securedprior to exposing the hip joint.

It is pointed out that the pelvic modular reference, in a preferredembodiment, is positioned while the patient is in supine decubitus.Moreover, as will be described hereinafter, the pelvic coordinate systemand table reference must also be digitized in supine decubitus. Afterthose manipulations, the patient can be repositioned in lateraldecubitus.

The femoral modular reference can be inserted at the proximal third fromthe femoral head of the femur or at the distal third from the femoralhead. These insertion points are examples, as any other suitable pointon the femur is considered. Positions of the trackable references are,for example, (1) looking posterior and towards the head, prior todislocation, and (2) a longer trackable reference, looking posterior,for the dislocated position. It is contemplated to use a single modularbase.

In Step 106, it is contemplated to digitize the coordinate system inlateral decubitus. It is also contemplated to collect postureinformation, as described in International Publication No. WO2004/030559 A1, by Jansen et al. Criteria may be used to validate thepoints taken and the computed surface.

In Step 106, a pelvic coordinate system and a femoral coordinate systemare digitized. In an embodiment, the pelvic coordinate system isdigitized with a registration pointer. In an embodiment, three pointsare taken on the pelvis 10 to create the frontal plane of the acetabularcoordinate system. Referring to FIG. 1, there is one point on the iliaccrest 12 of the operated side, one point on the contra lateral iliaccrest 13, and one point on one of the two pubic tubercles 14 of thepelvis 10. To be generally aligned, the points digitized on the iliaccrests 12 and 13 are taken at the outermost anterior point of the iliaccrests 12 and 13. The points digitized on the iliac crests 12 and 13 arepreferably taken directly on the soft tissue covering the bone pelvis onthe iliac crests, as the soft tissue is relatively thin thereon. Thepoint on the pubic tubercle 14 completes a first plane, the frontalplane. A second plane, the transverse plane, is perpendicular to thefrontal plane and includes the points on the iliac crests. A thirdplane, the sagittal plane, is perpendicular to the frontal andtransverse planes.

Supplemental information regarding the frontal plane can be obtained forvarious postures of a patient. For instance, trackable references can beused to gather information about sitting, standing and walking postures.This information can be used to adjust the orientation of the frontalplane, as these postures can provide information not available from thetypical lying posture in which a patient is during surgery. Thisinformation can influence the anteversion positioning of the implants.

It is possible to obtain anteversion and/or inclination values of theacetabulum of the patient, to be used as a reference (e.g., comparisonbasis) later in the surgery. To do so, points are digitized using aregistration pointer on the generally circular edge of the acetabulum 11and a plane is defined from these points. A normal to this plane and thepelvic frontal plane give the anteversion angle. The normal to thisplane is projected onto the acetabular frontal plane to give aninclination angle with a cranial-caudal axis.

For the digitization of the femoral coordinate system, it iscontemplated to collect five points of reference on the leg to the CASsystem, which is equipped with software that will create the femoralcoordinate system.

Referring to FIG. 1, a first point is taken on the tip of the greatertrochanter 23 of the femur 20, and will be defined as a starting pointof an anatomical axis of the femur 20. Thereafter, points are taken onthe medial and lateral epicondyles 24 and 25 of the femur 20,respectively. A midpoint between the medial epicondyle and lateralepicondyle points, in alignment therewith, is defined as an endpoint ofthe anatomical axis of the femur. The fourth and fifth points are takenon the medial malleolus 31 of the tibia 30 and on the lateral malleolus41 of the fibula 40, with the leg being bent at the knee. By having theleg bent at the knee, the tibia 30 stands on the posterior condyles 26of the femur 20. Therefore, an assumption is made wherein an alignedmidpoint of the medial and lateral malleoli points is said to define aplane (i.e., sagittal plane) with the anatomical axis, with an axis ofthe knee being normal to the sagittal plane. The frontal plane isperpendicular to the sagittal plane, with the anatomical axis lyingtherein. The transverse plane is perpendicular to the sagittal andfrontal planes, and can be positioned at any height. With the anatomicalaxis and the midpoint of the malleolus region digitized, the femoralcoordinate system, i.e., the femoral frame of reference, is complete. Itis noted that it is not required to measure two points to obtain amidpoint of the malleolus region. As this latter point will be in thesagittal plane, the only requirement is that a point is taken at amidpoint of the malleolus region, and may thus be placed approximatelyby the operator.

It is pointed out that the projection values described herein (e.g.,inclination, anteversion, etc.) are based on the acetabular and thefemoral coordinate systems. As it is contemplated to use alternativemethods of digitizing the acetabular and the femoral coordinate systems,in addition to the preferred methods of Step 116, the projection valueswould be related to the alternative acetabular and femoral coordinatesystem. For instance, another contemplated method for creatingcoordinate systems is described in U.S. Patent Application No.60/691,164, to Hodgson et al.

Other methods to gather information pertaining to surgical parametersare as follows. (1) The user digitizes a point on the greater trochanterbefore dislocation and retakes the same point, with the leg aligned inthe same orientation, after reduction. (2) The user digitizes a point onthe greater trochanter before dislocation and the system helps the userto replace the leg in the same orientation after reduction. The leglength and the offset are automatically computed when the leg ispositioned in range of the initial position before dislocation. (3) Theuser digitizes many points near the greater trochanter beforedislocation, the center of rotation of the acetabulum as described inStep 112 and the same points after reduction. The system aligns thesepoints and computes the leg length and the offset. Also, in each case,the CAS system may help the operator in placing the leg in a requiredinitial position.

In optional Step 108, a relative position between the pelvis and thefemur is registered with respect to the trackable references. The leg issimply left in a straight position, to align with a longitudinal axis ofthe body, and a relative position is acquired between trackingreferences secured to their respective bones.

In Step 110, the femur is dislocated from the pelvis, so as to exposethe acetabulum 11 and the femoral head 21 and neck 22.

In Step 112, a center of rotation is digitized for the acetabulum, bytaking reference points on the surface of the acetabulum 11. Referringto FIG. 3, a surface digitizing tool used to digitize the referencepoints on the surface of the acetabulum is generally shown at 150.

The surface digitizing tool 150 has a flexible film 152. The flexiblefilm 152 is made of a grid forming a network of micro-sensor nodes 154.More specifically, the micro-sensor nodes are nodes changingcharacteristics as a function of the shape of the flexible film 152.Accordingly, the overall shape of the flexible film 152 is calculable bydetermining the interrelations between adjacent nodes 154.

Various configurations are considered for the flexible film 152.According to one embodiment, the flexible film 152 is formed of a seriesof micro-pipes aligned to form a grid support by a substrate such as atextile or plastic film. Each micro-pipe contains an electrolyte (e.g.,NaCl, or like biocompatible electrolytes), varying in electricalcharacteristics (e.g., resistivity, capacity) as a function of pressuresustained by the micro-pipe (e.g., torsion resulting from thedeformation of the grid to match a surface). Each micro-pipe is dividedin a plurality of micro-pipe sections wired to allow the detection ofany variation in the electrical characteristics of the micro-pipesections as a result of shape variations. As the position of eachmicro-pipe section is known, it is then possible to generate a 3D modelfrom the calculated shape variations.

In the micro-pipe embodiment, the size and spacing between micro-pipesare selected as a function of the resolution required for the 3D modelof the object. As an example, spacing ranging between 0.5 to 1.0 mmbetween adjacent micro-pipes is sufficient to obtain a suitableresolution for a femoral head of a diameter of 60 mm, in the event thatthe flexible film 152 is used in hip replacement or resurfacing surgery.

In another embodiment, the film is formed fs strands made of a substratehaving variable characteristics when curved or bent. As an example,quartz crystal is a suitable substrate as its electrical characteristicsvary when subjected to pressure. Accordingly, it is considered toprovide the flexible film 152 made of quartz crystal substrate with amesh of electrical wires capturing the electrical characteristics, andvariations thereof, for different sections along the strand.

In another embodiment, the flexible film 152 is a metallic filmassociated with a mesh of electrical wires capturing the variations inelectrical characteristics at predetermined locations on the metallicfilm.

In another embodiment, the flexible film 152 has a mosaic ofmicro-elements arranged in a predetermined pattern, an example of whichis generally shown in FIG. 6, with micro-elements being illustrated by152′ having rectangular shapes (amongst other considered shapes), withthe micro-elements aligned in a quasi-uniform pattern (single ormultiple layers), and sandwiched between charged plates. Eachmicro-element 152′ has a positive end and a negative end, with thepolarity being proportional to the pressure sustained by the flexiblefilm as a function of its deformation. Scans are performed in bothdirections of alignment, whereby a 3D model can be digitized from theresult of both scans.

In another embodiment, the flexible film 152 is a mesh of opticalfibers, with Braggs gratings distributed along each optical fiber. Thelight captured at the exit of the optical fibers provides information onthe deformation of the flexible film 152. A 3D model can be created byassociating the positions of the Braggs gratings in the optical fibersto the deformation information obtained from the captured light.

In order to determine the position of each node, the flexible film 152is connected to a model generator, as will be described in furtherdetail hereinafter, which will calculate the 3-dimension shape of theflexible film 152 by determining the position of each node 154, to forma mesh of points.

In order to calibrate and relate the 3-dimensional model obtained fromthe flexible film 152 to a coordinate system, a trackable reference 156is secured to the flexible film 152 in a known relation. In the exampleof FIG. 3, the trackable reference 156, a passive trackable reference,is a known optical pattern, so as to be tracked for position andorientation by a CAS system. Therefore, a resurfacing processing unitcalculates the position and orientation of the 3-dimensional model withrespect to the frames of reference by relating the points of the nodes154 to the trackable reference 156.

As an alternative to the trackable reference 156, it is contemplated toprovide an active tracker connected to the flexible film 152. Forexample, magnetic (e.g., electro-magnetic), infrared and RF emitters areconsidered for use as reference 156, provided the use of suchtechnologies is acceptable in emergency-room environments.

In order to have the flexible film 152 take the shape of the acetabulum,it is contemplated to cover a generally spherical resilient member withthe flexible film 152. The combination of the flexible film 152 andresilient member is then fitted into the acetabulum, at which point theresilient member exerts an outward pressure forcing the flexible film152 to take the shape of the acetabulum.

The use of the surface digitizing tool 150 is advantageous in that thepositions of points are gathered in one step, and therefore represent aneconomy of time. Moreover, as points are currently digitized one by oneusing a pointer, the risk of handling error is increased. The resolutionof the flexible film 152 is typically controlled and tested during itsmanufacturing. The flexible film 152 is either disposable orsterilizable for further uses. The surface digitizing tool 150 is notlimited to being used in resurfacing surgery, but may be used in aplurality of surgical procedures in which it is desired to digitize bonemodels. Moreover, it is also considered to use the surface digitizingtool 150 along with a processing unit to generate digital models ofobjects other than anatomical parts. In many cases, the digital modeldoes not need to be related to a position and orientation, whereby thesurface digitizing tool 150 is not necessarily provided with thetrackable reference 156.

A center calculator (e.g., sphere fitter algorithm) is used to find theacetabular center of rotation from the 3-dimension shape obtained, andwill be described hereinafter with the description of a hip resurfacingCAS system. The acetabular center of rotation is therefore known as afunction of the tracking reference on the pelvis 10. In order to ensureprecise results, it may be required that a predefined number of pointsbe taken until validation criteria are met. Visual validation of thesphere found by the algorithm can also be performed. The center ofrotation and the diameter found may be displayed. Points are digitizedin the fossa (depth of the acetabulum). If the center of rotation of theacetabulum is known, it is not necessary to digitize the center ofrotation of the femoral head. However, it can be done without departingfrom the spirit of the present embodiment.

In Step 114, the acetabulum is altered in view of accommodating theacetabular cup implant. In order to guide the operator in altering theacetabulum, reamer position and orientation information is preferablyprovided, such that an axis of actuation of the reamer is for instancevisually displayed. The previous acetabular center of rotation is knownas a function of the tracking reference secured to the pelvis 10, as itwas acquired in previous Step 112. Preferably, the reamer is tracked forposition and orientation.

Examples of information that can be provided to the operator are asfollows: generic 2D images, mosaic or mesh in 3D viewers along withdrive shaft/reamer assembly in real time and/or display targeting viewsto help the user to align with target values, frontal and lateral views,inclination, inclination adjusted with the pelvic tilt, anteversion,anteversion adjusted with the pelvic tilt angles in real time, 3Dposition of the reamer center of rotation relatively to the acetabulumcenter of rotation, the distance between the reamer pole and acetabularwall.

The diameter of the pelvic implant chosen by the surgeon can be used todisplay a position of the new acetabular center of rotation incomparison to the digitized acetabular center of rotation (Step 112).For instance, the distance between the centers of rotation can bedisplayed numerically (e.g., in mm) as a function of the acetabularcoordinate system digitized in previous Step 106. Also, the anteversionand inclination of the actuation axis of the reamer, both as a functionof the acetabular coordinate system, can be given numerically (e.g., indegrees) to guide the surgeon in the reaming. More precisely, theanteversion is calculated as the angle between the axis of the reamerand the pelvic frontal plane, and the inclination is the angle betweenthe reamer axis projected onto the acetabular frontal plane and acranial-caudal axis (Step 106).

Step 116 consists in the insertion of the pelvic implant in theacetabulum 11, but it is pointed out that this step can also beperformed once the femoral head implant has been secured to the femur,according to the preference of the operator. A tracked impactor ispreferably used. As the pelvic implant size is known, the diameterthereof and the known relation between the impactor and the pelvicimplant is used with the tracking of the impactor to give theanteversion and the inclination of the pelvic implant. Also, thedistances between the current and the digitized centers of rotation canbe displayed. Therefore, the surgeon is guided during the use of theimpactor so as to position the pelvic implant to a given position of thecenter of rotation thereof, and to a given orientation [with respect toanteversion and inclination] to provide a maximal range of motion andstability of the leg.

Although the pelvic implant is secured at this point to the pelvis 10,it is possible to adjust the position and orientation of the pelvicimplant. Firstly, the tracked impactor, handle or like tool may bereconnected to the pelvic implant to serve as a lever in manipulatingthe pelvic implant with the tracked impactor, allowing position andorientation information (e.g., anteversion and inclination) to becalculated from the tracking of the impactor. Alternatively, points onthe circular edge of the pelvic implant may be digitized to define aplane, with the normal to this plane being used to calculate theanteversion and the inclination, as suggested previously to obtain thisinformation for the acetabulum.

Information typically provided with the use of the impactor includes:Display of generic 2D images, mosaic or mesh in 3D viewers along withimpactor/cup assembly in real time and/or display targeting views tohelp the user to align with target values, frontal and lateral views,navigation of the impactor and cup, display of inclination, inclinationadjusted with the pelvic tilt, anteversion, anteversion adjusted withthe pelvic tilt angles in real time, display of the 3D position of thecup center of rotation relatively to the acetabulum center of rotation.

In Step 118, a bone model is digitized for the femoral head 21 and neck22. The surface digitizing tool 150 is preferably used to create a3-dimensional model of the femoral head 21 and neck 22. In thisembodiment, it is preferred to simply cover the femoral head 21 and neck22 with the flexible film 152.

As tracking references have been secured to the femur 20 and the pelvis10 in Step 104, the points on the surface of the femoral head 21 areknown as a function of the tracking of the respective tracking referenceof the femur 20. As will be described hereinafter, a digital model ofthe femoral head and neck is produced, and may be displayed visually bythe hip resurfacing CAS system.

It is pointed out that the neck/head connection is preferably identifiedin the digital model of the femoral head and neck. Informationpreferably obtained includes the lateral aspect of femur at the greatertrochanter and the following 10 cm distally (as far as possible),internal aspect of femur at the lesser trochanter and the followingdistal region, and femoral neck itself (varus/valgus, anteversion). Thehead-neck junction is digitized or computed based on the points taken.If points are acquired automatically, collection of points can be takenby painting the femur. If points are acquired to build a mesh, pointsare taken on all the surface of the femur and not only on the frontaland transverse plane. The mesh can be constructed while points areacquired so users may take more points to have a more precisereconstruction.

The center of rotation of the femoral head may also be calculated fromthe digital model, for instance using a sphere fitter algorithm. If thecenter of rotation of the acetabulum is known, it may not be necessaryto digitize the center of rotation of the femoral head.

In Step 120, the desired guide orientation is determined. Morespecifically, the resurfacing of the femoral head will be dependent onthe orientation of a guide wire. Therefore, computer assistance isprovided to the operator so as to orient the guide wire in view of thesubsequent resurfacing of the femoral head.

Referring to FIG. 5, a drill guide positioning frame is generallyillustrated at 170. The drill guide positioning frame 170 is provided tofacilitate the planning of the desired guide orientation, and to guidethe drilling operation.

The drill guide positioning frame 170 has legs 172 supporting an annularsupport 174. The three legs 172 illustrated in FIG. 5 are displaceableand securable to the annular support 174, so as to hold the support 174fixed about the femoral head 21 in the manner illustrated in FIG. 5.

In the illustrated embodiment, the legs 172 each have an abutment end176 that will contour a part of the femoral neck 22. The abutment ends176 may be brought toward a common center by actuation of the lockabledegrees-of-freedom (DOF) between the legs 172 and the annular support174 so as to clamp onto the femoral neck 22. In a preferred embodiment,two translational DOFs are provided between the combination of the legs172 and the support 174, such that the support 174 is displaceable inits plane. For instance, an actuator 177 is provided to actuate bothtranslations DOFs.

In another embodiment, flexible film such as described in the surfacedigitizing tool 150 is provided on the abutments ends 176. In such acase, the flexible film is used to obtain the surface model of thefemoral neck 22, at the surfaces of contact between the positioningframe 170 and the femoral neck 22. In such an embodiment, the positionand orientation of the positioning frame 170 is tracked so as to relatethe 3-dimensional surface data calculated from the flexible film to theframe of reference of the femur.

An alignment tube 178 is generally centrally positioned in the annularsupport 174. The alignment tube 178 is supported to the support 174 by aspherical joint 180, so as to be displaceable in two rotational DOFs.The two rotational DOFs are lockable, so as to set a desired orientationfor the alignment tube 178. The alignment tube 178 is sized so as toaccommodate a drill and like tools having an elongated stem. Therefore,the drill received in the alignment tube 178 is displaceable axiallyalong the tube 178 so as to perform a drilling action.

In an embodiment, the support 174 is displaced in its plane bydisplacement with respect to the legs 172 so as to have the alignmenttube 178 in a suitable approximate position with respect to where aguide hole must be drilled into the femoral head 21. The translationalDOFs are then locked, in such a way that the only actuatable DOFs arethe rotational DOFs of the spherical joint 180.

The drill or like registration tool is received in the alignment tube178, and its longitudinal axis is tracked. Accordingly, the orientationof the drill is adjustable by the movement of the drill in the tworotational DOFs of the spherical joint 180. The support 174 may also bereleased from its locking relation with the legs 172 to adjust theposition of the alignment tube 178 in the plane of the support 174.

Once a desired orientation of the alignment tube 178 is reached asobtained from the racking of the drill or like tool inserted in thealignment tube 178, the DOFs are locked whereby the drilling step may beperformed.

It is pointed out that it is contemplated to motorize all or some of theDOFs of the drill guide positioning frame 170 to enable precisepositioning of the alignment tube 178 with respect to the femoral head21.

In order to plan the orientation of the guide wire, various views areprovided such as the frontal and top views of the reconstructed femur. Atemplate of the femoral implant over the femur model is also provided,as well as the following information: the initial CCD and anteversionangles, an initial template position, orientation and size with respectto the femoral center of rotation. The CCD is calculated as the anglebetween the projection of the guide wire on the femoral frontal planeand the longitudinal axis of the femur. Widgets are provided on screento translate and rotate the template in each view. Selectors areprovided to set the size of the implant and the neck diameter. If noflexible film is used in the positioning frame 170, the neck diameter isfound by two moving lines parallel to the template axis on the digitizedbone model. When the lines are on the contour of the neck, the diameteris determined. The CCD and anteversion angles are computed and displayedwhile the user is positioning the template. It is also contemplated toprovide means to rotate the model so it can be viewed in 360 degrees.Implant position, orientation and size are computed and suggested to theoperator as information to consider. Information that is preferablycomputed and displayed includes: the estimated range of motion, theestimated final leg length and offset, a graphical representation of thefemoral preparation (final result). Potential dislocation and/orimpingement is identified based on the cup position and orientation andthe planned position and orientation of the femoral implant. If thefemur is reconstructed with a mesh, the percentage of coverage may beprovided. Indications of where notching may happen should also beprovided.

In Step 122, the femur is altered for the insertion of the guide wire,using the positioning frame 170 as described previously (FIG. 3). Inorder to guide the operator in positioning and orienting the guide wireas planned, various information is provided, such as: generic 2D images,mosaic or mesh in 3D viewers along with guide wire/drill guide in realtime and/or display targeting views to help the user to align withplanned values, frontal and top views of the reconstructed femur,navigation of the guide wire with a drill guide, the CCD and anteversionangles, alignment views of the guide wire tracked with the drill guideon the CCD and anteversion axis found during the planning phase(aligning “bull's-eyes” or axes), the CCD and anteversion angles of theguide wire, audio and/or visual cues to let the operator know he/she is“in range” near the targeted angles by the means, the depth of the guidewire so the operator will be able to determine when the tip of the guidewire is near the lateral cortex of the proximal femur, potentialnotching with audio and/or visual feedback, and where this notchingcould potentially occur.

The same information can be provided for the insertion of a cannulateddrill guide, with a display of the depth of drilling so the user will beable to determine when to stop drilling according to the chosen implantsize.

Haptic devices can be used to ensure that the drilling only occurs whenthe orientation of the guide wire is as planned.

In Step 124, the femoral head 21 is resurfaced, by way of a reamer. Itis contemplated to provide visual information to the operator at thisstep. However, the guides inserted in the femur ensure that the reamingfollows planning. It is preferred that the operator keeps inspecting theactual femur especially during the cylindrical reaming, so as to avoidnotching of the femoral neck 22. Information that can be provided is asfollows: Tracking for position and orientation of the cylindricalreamer, generic 2D images, mosaic or mesh in 3D viewers along withcylindrical reamer in real time, frontal and top views of thereconstructed femur, navigation of the cylindrical reamer to track thereamed depth, orientation and position, the CCD and anteversion angles,a graphical representation of the result of the reaming, a pre-notchingwarning system based on probability to notch the cortex when theinstrument is close to it.

For the planar reaming, information that can be provided is as follows:generic 2D images, mosaic or mesh in 3D viewers along with planar reamerin real time, frontal and top views of the reconstructed femur, trackingof the planar reamer to track the reamed depth, orientation andposition, the CCD and anteversion angles, the distance between thehead-neck junction and the plane surface of the planar reamer,indications to the operator to stop reaming based on the selectedimplant size, how much bone has been removed, the leg length and theoffset based on the position of the planar reamer, a graphicalrepresentation of the result of the reaming, pre-notching warning systembased on probability to notch the cortex when the instrument is close toit.

In Step 126, the femoral implant is secured to the resurfaced femoralhead. Information that can be provided is as follows: position andorientation of the femoral component, generic 2D images, mosaic or meshin 3D viewers, frontal and top views of the reconstructed femur,navigation of the cement mantel to track the position and theorientation of the implant, the distance between the implant and theplane surface of the femur, the leg length and the offset. It iscontemplated to provide the possibility to attach the femoral implantwhile in place.

Although not illustrated in the method, there is provided thepossibility to ream again the acetabulum after the placement of thefemoral component if the initial reaming is not adequate, following theoptions provided in Step 114. Also, Step 116 could be performed at thispoint. Information that can be provided includes: the leg length and theoffset based on the position of the reamer relatively to the acetabulumcenter of rotation and the position and orientation of the femoralimplant with respect to the femur.

In the event that the acetabular cup is implanted at this point, theinformation that can be provided is as follows: tracking of the cupimpactor, generic 2D images, mosaic or mesh in 3D viewers along withimpactor/cup assembly in real time and/or display targeting views tohelp the user to align with target values, frontal and lateral views,display inclination, inclination adjusted with the pelvic tilt,anteversion, anteversion adjusted with the pelvic tilt angles in realtime, 3D position of the cup center of rotation relatively to theacetabulum center of rotation, the leg length and the offset based onthe position of the impactor relatively to the acetabulum center ofrotation and location of the femoral component on the femur.

In Step 128, an analysis of range of motion is performed. Information iscalculated, such as the range of motion of the joint after reduction,inclination, rotation and flexion/extension, possible dislocation (i.e.,detect if the center of rotation has moved) and/or impingement.

Referring to FIG. 4, a hip resurfacing CAS system is generally shown at200. The CAS system 200 has a resurfacing processing unit 201. Theresurfacing processing unit 201 is typically a computer or like devicehaving a processor.

Peripherals are provided in association with the resurfacing processingunit 201. In view of the trackable references 202 that will be securedto the femur and pelvis to define frames of reference (Steps 104 and106) and to the tracked tools 204 used throughout the method 100,tracking apparatus 206 is connected to the processing unit 201. Thetracking apparatus 206 is provided to track the trackable references 202and the tools 204 in the selected surgical environment. The trackingapparatus 206 may be any of optical sensors, RF sensors, magneticsensors and the like used in CAS systems.

Interface 207 is connected to the processing unit 201. The interface 207enables data entry and communications from the operator/surgeon of thesystem 200 to the processing unit 201. For instance, the interface 207may be a keyboard, mouse and/or touch screen or the like.

A display unit 208 is connected to the processing unit 201. The displayunit 208 provides information to the operator/surgeon throughout thesteps of the method 100. The data may be in the form of numericalvalues, as well as virtual representations of bone models along withsimulations of tools. Further detail about the data displayed by thedisplay unit 208 will be given hereinafter.

The resurfacing processing unit 201 has a CAS controller 210. The CAScontroller 210 is connected to the tracking apparatus 206 and to theinterface 207, so as to receive information therefrom. Morespecifically, the CAS controller 210 receives tracking data from thetracking apparatus 206, which tracking data will be interpreted by theprocessing unit 201. The CAS controller 210 receives user commands givenby the operator of the system 200 using the interface 207, andessentially controls the flow of information between the peripherals 206to 208, and between the other components 212, 214, 216, and 218 of theresurfacing processing unit 201. The CAS controller 210 performs certaintasks as well, such as calibration of tools.

The CAS controller 210 is also connected to the display unit 208. TheCAS controller 210 provides display data, in the form of numericalvalues and visual representations, to the display unit 208. The displayunit 208 displays this information.

A position/orientation calculator 212 is connected to the CAS controller210. The position/orientation calculator 212 receives the tracking dataof the tracking apparatus 206 from the CAS controller 210. Theinformation provided to the CAS controller 210 by theposition/orientation calculator 212 is in the form of theposition/orientation of a selected item of the trackable references 202or tools 204. For instance, following the method 100, the data providedby the calculator 212 may be the pelvic and femoral coordinate systemsfrom the trackable references 202. As another example, the data takesthe form of a real-time orientation of the operating axis of one of thetools 204, such as the axis of a reamer, or a real-time position of atip of one of the tools 204, such as a registration pointer.

A center calculator 214 (i.e., a surgical parameter calculator 214) isassociated with the CAS controller 210. The center calculator 214 isprovided to digitize the center of rotation of the pelvis (as describedfor Step 112) and the center of rotation of the femoral head (optionallyin Step 118). The center calculation is performed using theposition/orientation data calculated by the position/orientationcalculator 212, as well as commands from the CAS controller 210.

In the embodiment of FIG. 3, the center calculation is performed usingthe surface digitizing tool 150 and/or the positioning frame 170 whichprovide meshes of points representing the surface of the acetabulum(Step 112), of the femoral head and neck (Step 118), and of the femoralneck (Step 122). An indication that the center calculation is to beperformed by the center calculator 214 is commanded by the CAScontroller 210, for instance as a response to a command from theoperator using the interface 207. The position of the centers istherefore calculated with respect to the coordinate systems (Step 106),and the information is updated in real-time by the CAS controller 210.

A model generator 216 is associated with the CAS controller 210 and withthe position and orientation calculator 212 in a model generator system.The model generator 216 receives the signals representing the meshes ofpoints from the flexible film (of the tool 150 or the frame 170) incombination with commands from the CAS controller 210, following Steps112, 118 and 122. The model generator creates 3-dimensional models fromthese signals, and combines the model to the position and orientation ofthe trackable member 156 to relate the bone model to the frames orreference. For instance, in Step 118, a surface model of the femoralhead and neck is obtained. The surface model is associated with thecoordinate systems obtained from the tracking of the trackablereferences 202.

More specifically, the model generator 216 obtains a signal from each ofthe micro-sensor elements when the film is shaped to model the femoralhead and neck and/or the acetabulum. The signals are used, along withthe individual position of each of the micro-sensor element, todetermine a shape variation of the film at each of the micro-sensorelement. Subsequently, the 3D model of the bone element (i.e., femoralhead/neck, acetabulum) is generated from the shape variation and theindividual position of each of the micro-sensor elements.

A resurfacing evaluator 218 is provided in association with the CAScontroller 210. The resurfacing evaluator 218 (is provided to determinethe evaluated bone resurfacing alteration, which is the effect of aresurfacing tool (from the tools 204) on the bone model. Accordingly,bone model data is provided by the model generator 216, along with theposition and orientation of a reaming tool as determined by the CAScontroller 210 from tool geometry data and an orientation of abone-altering tool (such as a drill) from the tools 204.

In the case of femoral head resurfacing, as the precision of the reamingmust be respected, it has been described previously that a guide wire isprovided, in order to drill a guiding bore in the femoral head prior toreaming. Therefore, the evaluated bone resurfacing alteration isindicated as a function of the orientation of the axis of the drillguide. Therefore, information associated with a potential wrongfulreaming is provided to the operator, such that the operator is guidedinto drilling the drill guide in a suitable orientation in view of theeffects on resurfacing. The resurfacing evaluator 218 may also be usedto calculate the effect of acetabulum reaming on associated data (pelviccenter of rotation, anteversion, etc.)

Throughout surgery, the display unit 208 provides the data discussedabove. For instance, the output of the model generator 210 is convertedby the CAS controller 210 to a virtual model of the bone surface to bealtered, for instance with virtual real-time representations of thetools with respect to the bone models. Accordingly, warning can besignaled to the operator/surgeon if the effects of resurfacing areoutside acceptable standards. Again, in femoral head resurfacing, thefemoral neck must not be nicked, whereby drill guide axis data can beassociated with a warning signal to guide the operator/surgeon inadjusting the orientation of the drill.

Moreover, numerical information is also provided to the operator, whichnumerical information is described previously for the steps of themethod 100.

Various instruments can be used, such as blunt tracked pointers(straight or curved), adapted to fit on a rotational tracker or auniversal handle to paint bones (acetabulum, femur, etc.). The drillguide or guides can be designed to fit on a universal handle or arotational tracker. A mechanism may be used to block/hold the positionand the orientation of the drill guide. Planar reamer is modified to beused in conjunction with the rotational tracker. Technology to haveappropriate drilling instrument if the user wants to navigate the drillbit only.

In other contemplated options there are the possibility to navigate theguide wire, the guide wire and the cannulated drill bit or only thedrill bit, the possibility to rotate, translate and zoom the viewers,the animation or illustration to describe to the operator the upcomingtasks, the possibility to take snapshots, menus allowing selection ofoptions and parameters during the procedure, allowing navigating throughthe surgical steps in the application, step-driven (wizardlike sequenceof pages), status icons to display tracking state of an instrument,volume view/aim camera to display in space the location of the trackersseen by the camera, give information on the tracked state of a tracker(out of volume, missing sphere, IR interference, etc).

1. A model generator system for generating models of objects,comprising: a surface digitizing tool having a film of micro-sensorelements, with each micro-sensor element related in a network affectedby a shape of the film, the film being flexible to conform to a shape ofa selected surface of an object; a processing unit for receiving signalsfrom the micro-sensor elements of the network, the processing unithaving a model generator producing a model of the selected surface ofthe object from the signals of the network of resistive elements.
 2. Themodel generator system according to claim 1, further comprising atracking apparatus, and wherein: the surface digitizing tool has atrackable reference connected to the film in a known relation, thetrackable reference being tracked by the tracking apparatus; and theprocessing unit receives tracking data for the trackable reference andfurther comprises a position/orientation calculator to calculate fromthe tracking data a position and orientation of the trackable reference,with the model generator receiving said position and orientation data ofthe trackable reference to associate the position and orientation datato the known relation between the film and the trackable reference toprovide a position and orientation of the selected surface of theobject.
 3. The model generator system according to claim 2, wherein thetracking apparatus is a passive optical tracking apparatus, and thetrackable reference is a selected pattern of retro-reflective spheres.4. The model generator system according to claim 1, wherein the objectis a bone element and the model generator system is used incomputer-assisted surgery.
 5. The model generator system according toclaim 4, wherein the surface digitizing tool has a resilient body, thefilm of micro-sensors being mounted on the surface of the resilientbody, the surface digitizing tool being used to conform to the shape ofa bone element cavity with the resilient body deforming to enter thecavity and applying resilient forces against a surface of the cavity,whereby the film of micro-sensors conforms to the shape of the cavity.6. The model generator system according to claim 5, wherein the boneelement cavity is an acetabulum.
 7. The model generator system accordingto claim 4, wherein the surface digitizing tool is used with a boneelement having a generally semi-spherical shape, and the processing unithas a center calculator for receiving the model of the selected surfaceof the bone element and determining a center of the bone element fromsaid model.
 8. The model generator system according to claim 7, whereinthe bone element is any one of an acetabulum and a femoral head.
 9. Themodel generator system according to claim 1, wherein the micro-sensorsare resistive elements.
 10. The model generator system according toclaim 1, wherein the micro-sensors are capacitive elements.
 11. Apositioning frame for aligning a position and orientation of a drillingtool with respect to a bone element, comprising: a connector portionadapted to releasably grasp the bone element; a support portionoperatively connected to the connector portion and positioned withrespect to a portion of the bone element to be drilled; an alignmenttube operatively connected to the support portion, the alignment tubebeing adapted to receive a working end of a drill to align the drillwith the bone element; and joints between the alignment tube and theconnector portion to adjust a position and an orientation of thealignment tube with respect to the bone element.
 12. The positioningframe according to claim 11, wherein the joints comprise at least onejoint providing at least two translational degrees of freedom betweenthe connector portion and the alignment tube to adjust a position of thealignment tube, the two translational degrees-of-freedom being lockable.13. The positioning frame according to claim 12, further comprisingdegrees of actuation to control and actuate the at least twotranslational degrees of freedom.
 14. The positioning frame according toclaim 11, wherein the joints comprise at least one joint providing atleast two rotational degrees of freedom between the connector portionand the alignment tube to adjust an orientation of the alignment tube,the two rotational degrees-of-freedom being lockable.
 15. Thepositioning frame according to claim 11, wherein ends of the connectorportions grasping the bone element each have a film of micro-sensorelements, with each micro-sensor element related in a network affectedby a shape of the film, the film being flexible to conform to a shape ofthe bone element so as to provide data pertaining to a geometry of thebone element.
 16. The positioning frame according to claim 11, whereinthe bone element is a femur, and the connector portion releasably graspa femoral neck of the femur, and the alignment tube is aligned with afemoral head of the femur.
 17. The positioning frame according to claim11, wherein the joints provide four degrees of freedom between thealignment tube and the connector portion, with at least two degrees ofactuation to actuate and control two selected ones of the degrees offreedom.
 18. The positioning frame according to claim 11, wherein thesupport portion has annular body, with the alignment tube being heldgenerally in a center of the annular body, and wherein the connectorportion has three legs projecting from the annular body.
 19. A methodfor drilling a bone element in computer-assisted surgery, comprising:providing a positioning frame having a drill guide with adjustabledegrees of freedom for the drill guide in the positioning frame, and acomputer-assisted surgery system providing orientation data associatedwith tracking of the drill guide and of a frame of reference of the boneelement; clamping the positioning frame to the bone element such thatthe drill guide is in the vicinity of a portion of the bone element tobe drilled; displacing the drill guide along the degrees of freedom as afunction of the orientation data from the computer-assisted surgery,until a desired orientation is reached for the drill guide; and drillingthe bone element by passing a drill bit through the drill guide in thedesired orientation.
 20. The method according to claim 19, whereindisplacing the drill guide comprises inserting a drill bit of a drill inthe drill guide and moving the drill and the drill guide concurrently.21. The method according to claim 20, wherein providing orientation datacomprises calculating the orientation data as a function of a trackingof the drill.
 22. The method according to claim 19, wherein displacingthe drill guide comprises locking the degrees of freedom at the selectedorientation.
 23. The method according to claim 19, wherein displacingthe drill guide comprises actuating degrees of actuation to actuate thedegrees of freedom.
 24. The method according to claim 19, whereinclamping the positioning frame comprises clamping the positioning frameto a femoral neck, such that the drill guide is in the vicinity of afemoral head as the femoral head is drilled.
 25. A method for generatinga model of an object comprising: providing a film of micro-sensorelements, with an individual position in the film of each saidmicro-sensor element known; obtaining a signal from each saidmicro-sensor element when the film is shaped to model an object;determining a shape variation of the film at each said micro-sensorelement from the signal; and generating a model of the object from theshape variation and the individual position of each said micro-sensorelement.
 26. The method according to claim 25, wherein providingcomprises providing a trackable reference connected to the film ofmicro-sensor elements, with a relation between the film and thetrackable reference known, and further comprising tracking the trackablereference, and calculating a position and orientation of the object fromthe model of the object and the tracking of the trackable reference. 27.The method according to claim 25, wherein obtaining comprises obtaininga signal from each said micro-sensor element when the film is shaped tomodel a bone element in computer-assisted surgery.
 28. The methodaccording to claim 27, wherein obtaining comprises obtaining a signalfrom each said micro-sensor element when the film is shaped to model atleast one of a femoral head and an acetabulum in computer-assisted hipreplacement surgery.